Stirling Cycle engines, sometimes called Hot-air engines or Hot-gas engines, are, as evidenced by more than 600 patents since its invention in the early 19th century by Reverend Robert Stirling of Edinburgh, Scotland, from whom the thermodynamic cycle gets its name, a well known genre of machine chiefly characterized by an operating process in which an internally-contained working fluid is alternately and periodically heated and cooled, via conduction, through heat-exchangers which are integral parts of the machine, by an external heat source and an external heat sink, in order to cause cyclic pressure changes within the working fluid and thereby accomplish work. These heat-exchangers are generally closed cylinders, or closed spaces of other geometry, with variable volumes. The variation in volume is accomplished by means of sliding pistons or other movable members which are usually connected to a common crankshaft, though multiple crankshafts and other mechanical arrangements also exist, which maintain a phase angle between the pistons and/or displacers. Some of these machines have a mechanical arrangement in which a compression piston and a displacer piston reside within a single cylinder, and are called co-axial machines. In others, the pistons are arranged in multiple, inter-connected cylinders, in which the pistons act upon the working fluid in both directions of stroke, and are therefore called double-acting machines. Still in other machines, sometimes referred to as Wankel or rotary engines, the working mechanism consists of multiple, tangential-displacement rotors which act upon the working fluid to effect its movement between heat-exchangers.
The common relationship between these machines is that the working cycle necessitates that the related cold-side variable volume and the hot-side variable volume be so connected that said volumes vary in a fixed phase relationship to one another, of approximately ninety degrees. By this arrangement, a working fluid within two such, connected, variable-volume spaces can be compressed or decompressed by pistons or other displacement mechanisms which are connected to a common crankshaft. When the heat exchangers are properly designed, the working fluid is increased in temperature and pressure so that it performs work upon the cold-side displacement mechanism through part of the thermodynamic cycle, then is decreased in temperature and pressure so rapidly that the cold-side displacement mechanism can be carried through the remainder of the cycle by inertial moment, without performing work upon the working fluid. In this way a net gain in work output is realized.